Radiosondes are customarily employed to measure atmospheric conditions including pressure, temperature and humidity. The radiosonde is airborne and, among other sensing elements, includes a pressure transducer in the form of a pressure sensitive diaphragm for measuring variations in pressure, a thermistor for measuring temperature and a carbon hygristor for measuring humidity. In the past, at least the diaphragm has been mounted with or without the other sensing elements as a separate assembly apart from the electronic circuitry required to convert the analog readings from the measuring or sensing elements into signals which can be transmitted to a remote ground station. In many cases, the radiosonde also incorporates a receiver to receive position information for windfinding, and these signals are transmitted to the ground station, also.
It has been heretofore proposed to measure temperature, humidity and pressure conditions in the atmosphere by electronic circuitry which will generate voltage signals representing the various parameters, and an analog-to-digital converter is employed to convert those signals into digital form which can be transmitted to a remote ground station. Typically, the converter is sequentially connected to the sensing elements through a multiplexer to receive the analog signals representing the sensed atmospheric conditions and convert same to digital form. A microprocessor interfaces with the converter to process the signals so converted and delivered to an RF transmitter which transmits the information in digital form to the remote ground station. Representative patents are disclosed U.S. Pat. Nos. 4,481,514 to J. M. Beukers et al; 3,715,638 to W. R. Polye; 4,040,118 to S. A. Johnston and 4,084,438 to S. Y. Lee et al.
It is a feature of the present invention to measure atmospheric conditions including temperature, humidity and pressure by time interval measuring circuitry for the direct and accurate conversion of resistance and capacitance values representing those conditions into binary numbers. The binary numbers are transmitted in serial digital form, with error detection codes, to a remote ground station. A first measuring circuit is employed to measure resistance values in such a way as to avoid placing the temperature and humidity sensors in an RC oscillator circuit so that very little electromagnetic energy is radiated by the wires which connect the sensors to the resistance measurement circuitry and is particularly important if the radiosonde contains a receiver for receiving position information or location signals. The measuring circuit should be capable of measuring resistance over a wide dynamic range which will match the requirements for a thermistor and carbon hygristor and avoid the problems associated with the use of RC oscillators which impose limits on frequency variation thereby resulting in losses in sensitivity.
A second measuring circuit is capable of measuring differential pressure or capacitance ratios in a pressure cell so that the effects of temperature, gain and component changes in the circuit do not affect its accuracy and greatly minimize the effects of stray capacitance on the accuracy of measurement. The time interval to be measured in each circuit is converted into a binary number by a common counter circuit associated with the system clock in the microprocessor and the entire system is program-controlled to order out the time intervals measured along with the necessary calibration coefficients for transmittal to a ground station. Another feature is to install the measuring and control circuitry on the surface of the ceramic that forms the substrate for a simplified form of offset diaphragm, pressure cell which obviates the extension of conductors or leads away from the interior of the diaphragm and results in an extremely compact assembly thereby avoiding the effects of stray capacitance.